1. Field of the Invention
The present invention relates to a method of forming carbon nanotubes, and more particularly, to the growth and purification of carbon nanotubes using plasma.
2. Description of the Related Art
Carbon, the most important constituent element, which is combined with oxygen, hydrogen, nitrogen and the like, of all organisms including the human body, has four unique crystalline structures including diamond, graphite, fullerene and carbon nanotubes. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall or multi-wall, which can be obtained by rolling up a sheet formed of a plurality of hexagons, the sheet formed by combining each carbon atom thereof with three neighboring carbon atoms. The carbon nanotubes have a diameter in the order of a few nanometers to a few hundred nanometers. Carbon nanotubes can function as either a conductor, like metals, or a semiconductor, according to the rolled shape and the diameter of the helical tubes. Also, its hollow structure with a predetermined length allows for good mechanical, electrical and chemical properties, so that carbon nanotubes are known to be a material for field emission devices, hydrogen containers and electrodes of rechargeable batteries.
Originally, carbon nanotubes produced by an arc discharge between two graphite rods was discovered and reported in an article entitled "Helical Microtubules of Graphitic Carbon" (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58) by Sumio lijima. This technique is commonly used to produce carbon nanotubes, however, yield of pure carbon nanotubes with respect to the end product is only about 15%. Thus, a complicated purification process must be carried out for particular device applications.
Another conventional approach to produce carbon nanotubes, which was described in an article entitled "Epitaxial Carbon Nanotube Film Self-organized by Sublimation Decomposition of Silicon Carbide" (AppI. Phys. Lett. Vol. 71, pp. 2620, 1977), by Michiko Kusunoki, is to produce carbon nanotubes at high temperatures by irradiating a laser onto graphite or silicon carbide. In this case, the carbon nanotubes are produced from graphite at about 1200.degree. C. or more and from silicon carbide at about 1600 to 1700.degree. C. However, this method also requires multiple stages of purification which increases the cost. In addition, this method has difficulties in large-device applications.
A method of producing carbon nanotubes through a thermal decomposition of hydrocarbon series gases by chemical vapor deposition (CVD) was reported by W. Z. Li et al. in an article entitled "Large-Scale Synthesis of Aligned Carbon Nanotubes" (Science, Vol. 274, Dec. 6, 1996, pp. 1701-1703). This technique is applicable only with a gas that is unstable, such as acetylene or benzene. For example, a methane (CH.sub.4) gas cannot be used to produce carbon nanotubes by this technique.